Oxygen reduction reaction catalyst and preparation and application thereof

Abstract

This invention discloses an oxygen reduction reaction catalyst, its preparation, and its application. The catalyst comprises a layered structure formed by an array of hollow carbon nanocages loaded with p-doped metal single atoms. The preparation method involves: dissolving a zinc salt and a second metal salt in water to obtain a metal source solution; mixing the metal source solution with an aqueous solution of 2-methylimidazole to obtain metal-organic framework nanoparticles; dispersing the metal-organic framework nanoparticles and a phosphorus-containing etchant capable of thermally decomposing to generate ammonia in water to form a mixed colloidal solution; freeze-drying the mixed colloidal solution to obtain a layered precursor; and calcining the layered precursor to obtain the oxygen reduction reaction catalyst. This catalyst can be applied to electrochemical oxygen reduction testing and the preparation of proton exchange membrane fuel cells and zinc-air batteries. The catalyst prepared by this invention exhibits good mass transfer performance and a high specific surface area, significantly improving the accessibility of active sites and possessing excellent electrocatalytic performance.

Description

Technical Field

[0001] This invention relates to a catalyst for electrocatalytic reactions and a method for preparing the same, and more particularly to an oxygen reduction reaction catalyst and its preparation and application. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) and zinc-air batteries (ZABs) have distinguished themselves in advanced energy technologies due to their high energy density and environmentally friendly properties. Recent research has prioritized transition metal single-atom catalysts, particularly M–N–C systems with high oxygen reduction reaction (ORR) activity, due to the high cost of platinum-based catalysts. To address the poor adsorption of oxygen intermediates caused by the symmetrical electron distribution within the M–N4 structure, numerous studies have employed strategies such as adjusting the asymmetric coordination environment of M–N–C or constructing bimetallic sites to improve the intrinsic turnover frequency (TOF) per active site. Of particular note is that the performance of electrocatalysts depends not only on their intrinsic activity but, more importantly, on the utilization efficiency (UE) of the active sites. However, improving UE presents a significant challenge because M sites are typically embedded in randomly stacked carbon particles with a dense carbon framework. Therefore, the construction of hollow catalysts with open porous structures has attracted increasing attention. This structure facilitates substrate permeation, thereby shortening the mass diffusion path in the ORR process and improving the utilization of active sites.

[0003] In recent years, zeolite imidazole framework (ZIF) nanocrystals have emerged as a promising candidate for the successful fabrication of various functionalized hollow carbon nanostructures using sacrificial templates. More recently, a three-dimensional (3D) ordered macroporous carbon support has been successfully fabricated using a polystyrene colloidal crystal template strategy. This support serves as a universal anchoring matrix for various active sites and has demonstrated excellent performance in multiple catalytic applications, thanks to its superior mass transfer efficiency within continuous open pores. However, this hard-templating method typically involves complex synthetic steps, stringent experimental conditions, and environmentally harmful template removal processes. Furthermore, anisotropic chemical etching is a promising approach for constructing macroporous structures, enabling directional control of etching at the nanoscale. To date, catechol orange, cyanuric acid, and acetic acid vapor have been used as etchants to generate hollow nanoframeworks with anisotropic morphologies. However, these etching methods involve multi-step processes and require strict control of pH and etching time to avoid over-etching. Moreover, open macroporous structures obtained through either hard-templating or chemical etching methods exhibit only localized interparticle synergies, limiting mass transport to short-range channels and failing to achieve optimal mass transport efficiency. The slow kinetics of ORR stem from its occurrence at the three-phase boundary (catalyst / electrolyte / gas), where limited oxygen solubility and hindered mass transport in the electrolyte further restrict the accessibility of active sites.

[0004] Despite the excellent performance of hollow carbon nanoparticle catalysts, the random stacking structure within the thick catalyst layer on the electrode still hinders efficient mass transfer and diffusion at the macroscopic scale. This contradiction highlights the limitations of existing strategies: 1. How to overcome the difficulty of uncontrollable etching degree caused by the multi-step operation of the current anisotropic etching strategy; 2. Overcoming the mass transfer barrier caused by the random stacking of porous particles has become the core challenge in constructing efficient mass transfer channels in heterogeneous electrocatalysis.

[0005] Therefore, developing strategies to generate catalysts with long-range interconnected open structures to enhance mass transport and active site utilization in ORR has become an urgent technical problem to be solved. Summary of the Invention

[0006] Objectives of this invention: The objective of this invention is to provide an oxygen reduction reaction (ORR) catalyst, addressing the problem of enhancing mass transport and active site utilization in ORR. A second objective is to provide a method for preparing the ORR catalyst, addressing the problem of preparing an ORR catalyst with a hollow carbon nanocage layered array structure. A third objective is to provide an application of the ORR catalyst in electrochemical oxygen reduction testing, addressing the problem of how to perform electrochemical oxygen reduction testing. A fourth objective is to provide an application of the ORR catalyst in the preparation of battery cathodes, addressing the problem of how to prepare battery cathodes.

[0007] Technical solution: The oxygen reduction reaction catalyst of the present invention comprises a layered structure formed by an array of hollow carbon nanocages loaded with P-doped metal single atoms.

[0008] Preferably, the metal includes at least one of Co, Fe, and Cu, and the carbon nanocage is in the shape of a cube, a rhombic dodecahedron, or a chamfered rhombic dodecahedron, and the hollow carbon nanocage contains micropores, mesopores, and macropores.

[0009] This invention effectively solves the problem of low utilization of active sites and difficulty in flexibly controlling the adsorption energy of reaction intermediates (such as the adsorption of OOH* in ORR) caused by the geometric symmetry limitation of the adsorption of reaction intermediates in planar symmetric M-N4 single-atom catalysts due to the specific hollow carbon nanocage layered array structure.

[0010] The second aspect of this invention discloses a method for preparing the above-mentioned oxygen reduction reaction catalyst, comprising the following steps: (1) Dissolve zinc salt and second metal salt in water to obtain metal source solution, and then react the metal source solution with 2-methylimidazole aqueous solution to obtain metal-organic framework nanoparticles; (2) Disperse metal-organic framework nanoparticles and phosphorus-containing etchant that can thermally decompose to generate ammonia in water to form a mixed colloidal solution, and freeze-dry the mixed colloidal solution to obtain a layered precursor; (3) The oxygen reduction reaction catalyst is obtained by calcining the layered precursor.

[0011] This invention uses diammonium hydrogen phosphate (DAP) as the etching gas source and P-doped coordination P source, and metal-organic framework nanoparticles (M / Zn-ZIF-8, where M is the second metal element) as the single-atom metal source and carbon source template. The precursor is obtained after freeze-drying following ice template self-assembly. By changing the mass ratio of DAP to M / Zn-ZIF-8 and the type of the second metal M in the precursor, not only can an asymmetric P-coordination microenvironment be achieved, but also anisotropic etching is realized using the corrosive gas (NH3) released during DAP heating. A hollow carbon nanocage array electrocatalyst is prepared via a one-step pyrolysis method, overcoming the difficulty of uncontrollable etching degree caused by the multi-step operation required by current anisotropic etching strategies.

[0012] Specifically, during the low-temperature pyrolysis process, the NH3 gas generated from the thermal decomposition of DAP in the precursor acts as a Lewis base, selectively etching the {100} facets of ZIF-8 through high-density coordination bonds. Subsequently, during the high-temperature pyrolysis process, stress contraction drives a further hollowing process, forming a unique open-cell nanocage structure. Furthermore, the P source generated from the thermal decomposition of DAP achieves a coordination environment for single-atom P-doped metal, realizing synergistic control from microscopic coordination to macroscopic structure.

[0013] The hollow carbon nanocage array catalyst with P-doped single atoms prepared in this invention has an intrinsic activity enhanced by the asymmetric coordination of active centers at the microscopic level. The hollow carbon nanocage array structure can accelerate electrolyte penetration and enhance mass transfer efficiency during electrocatalysis, thereby improving the utilization rate of active sites.

[0014] Preferably, in step (1), the second metal salt is at least one of the acetates of Co, Fe, and Cu, the zinc salt is zinc acetate, and the 2-methylimidazole aqueous solution is an aqueous solution containing 2-methylimidazole and a quaternary ammonium salt surfactant.

[0015] In some embodiments, the quaternary ammonium salt surfactant is hexadecyltrimethylammonium bromide. By changing the type of the second metal M, the coordination environment of different metal single atoms in P doping can be controlled, overcoming the difficulty that most heteroatom doping methods can only control the coordination environment in one way, thus achieving the synthesis of M / Zn-ZIF-8 crystals doped with different metal centers.

[0016] Furthermore, the molar ratio of zinc, the second metal, and 2-methylimidazole in the metal source solution is 10-15:0.5-1.5:50-150.

[0017] Preferably, in step (2), the phosphorus-containing etchant includes at least one of diammonium hydrogen phosphate and diammonium dihydrogen phosphate. Preferably, in step (2), the mass ratio of metal-organic framework nanoparticles to phosphorus-containing etchant is 200-400:5-25; this ratio is preferably 300:10-20.

[0018] This invention optimizes the etching degree of the corrosive gas (NH3) generated during the carbonization process of the phosphorus-containing etchant by changing the mass ratio of phosphorus-containing etchant and metal-organic framework nanoparticles in the precursor, thereby promoting the formation of hollow carbon nanocage structures and overcoming the difficulty of uncontrollable etching degree caused by the multi-step operation of the current anisotropic etching strategy.

[0019] Without the addition of phosphorus-containing etchant, the resulting carbon nanoparticles retain the rhombic dodecahedral morphology of truncated ZIF-8, forming a solid carbon nanoparticle array. With a low DAP content, only phosphorus-doped solid carbon nanoparticle arrays are obtained. However, with a high phosphorus-containing etchant content, the carbon nanocages collapse significantly, forming a collapsed nanocage array. By adjusting the phosphorus-containing etchant content, the etching degree can be controlled. Utilizing this gas-phase anisotropic etching process, a hollow carbon nanocage array electrocatalyst was prepared, overcoming the difficulty of uncontrollable etching degree caused by the multi-step operation of current anisotropic etching strategies, and possessing the advantage of wide applicability.

[0020] In step (3), the calcination conditions are: under an inert atmosphere, the temperature is raised to 500-1000 ℃, calcined at a constant temperature for at least 2 h, and then cooled to room temperature.

[0021] In some embodiments, the inert atmosphere is formed by continuously introducing argon or nitrogen gas at a flow rate of 30-100 mL / min into the calcining furnace.

[0022] Furthermore, the heating rate is 1-5 °C / min, and the cooling rate is 5-10 °C / min.

[0023] More preferably, the heating rate is 2-4 ℃ / min, the target temperature is maintained at 500-1000 ℃ for 2-4 hours, and the cooling rate is 5-10 ℃ / min.

[0024] The third aspect of this invention discloses the application of the above-mentioned oxygen reduction reaction catalyst in electrochemical oxygen reduction testing.

[0025] The fourth aspect of this invention discloses the application of the above-mentioned oxygen reduction reaction catalyst in the preparation of battery cathodes.

[0026] Specifically, the above application includes the following steps: 1-5 mg of the above oxygen reduction reaction catalyst is mixed with 10-50 μL of 1-10% Nafion solution and 100-1000 μL of ethanol, ultrasonicated until homogeneous, and then uniformly drop-coated onto carbon paper to obtain an air cathode.

[0027] In some embodiments, the battery includes a proton exchange membrane fuel cell and a zinc-air battery.

[0028] This invention utilizes the mass transfer performance provided by the hollow carbon nanocage array structure to improve the accessibility and utilization of single-atom active sites, thereby achieving high open-circuit voltage, peak power density, rate performance, specific capacity, and cycle stability of ZABs.

[0029] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1) This invention uses an ice-template self-assembled M / Zn-ZIF-8@DAP array precursor for carbonization. During low-temperature pyrolysis, the NH3 gas generated from the thermal decomposition of DAP in the precursor acts as a Lewis base, selectively etching the {100} facets of ZIF-8 with high-density coordination bonds. Subsequently, during high-temperature pyrolysis, stress contraction drives a further hollowing process, forming a unique open-cell nanocage structure. The catalyst prepared by this invention exhibits good mass transfer performance and high specific surface area, significantly improving the accessibility of active sites and demonstrating excellent electrocatalytic performance.

[0030] 2) The preparation method provided by the present invention is simple and efficient, overcoming the difficulty of uncontrollable etching degree caused by the multi-step operation of the current anisotropic etching strategy. The resulting catalyst has broad application prospects in the field of energy storage and conversion.

[0031] 3) The metal-doped M / Zn-ZIF-8@DAP precursor carbonization method can achieve different P-doped metal single atoms by changing the source of different metal carboxylates, thereby breaking the symmetry of planar M-N4 single atoms and controlling the center position of the d-band, which can significantly improve the activity and stability of single-atom catalysts and improve the adsorption energy of reaction intermediates.

[0032] 4) The hollow carbon nanocage array electrocatalyst supported on P-doped metal single atoms prepared by this invention has advantages such as tunable metal active centers, high intrinsic reaction activity, high specific surface area, and strong multiphase mass transfer ability. It also exhibits high reactivity and selectivity in ORR tests, ZABs, and PEMFCs. Attached Figure Description

[0033] Figure 1This is a transmission electron microscope (TEM) image (at 100 nm scale) of the hollow carbon nanocage array loaded with P-doped Fe single atoms prepared in Example 1. Figure 2 The image shows a high-angle annular dark field (HAADF-STEM) image of the hollow carbon nanocage array catalyst supported on P-doped Fe single atoms prepared in Example 1. Figure 3 Thermogravimetric curves of Fe / Zn-ZIF-8@DAP, Fe / Zn-ZIF-8, and DAP in Example 1 are shown. Figure 4 These are the products of DAP decomposition under different heating temperatures; Figure 5 The images show scanning electron microscope (SEM) images of hollow carbon nanocage array catalysts supported on P-doped Fe single atoms prepared at different carbonization temperatures in Example 1, and SEM images of catalysts prepared in Examples 2 and 3. Figure 6 This is a schematic diagram illustrating the mechanism of anisotropic etching achieved by the thermal decomposition of DAP to generate NH3 gas in Example 1. Figure 7 N2 adsorption isotherms of catalysts prepared under different conditions; Figure 8 This is a TEM image of the solid carbon nanoparticle array prepared in Comparative Example 1. Figure 9 The scan linear voltammetry (LSV) plots of the catalyst prepared in Example 1 and commercial Pt / C in ORR testing are shown. Figure 10 The discharge and power density curves of the catalyst prepared in Example 1 and ZABs prepared from commercial Pt / C are shown. Figure 11 The discharge and power density curves of PEMFCs prepared using the catalyst and solid carbon nanoparticle array obtained in Example 1 are shown. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0035] Example 1: A method for preparing a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms is as follows: (1) Preparation of tangential rhombic dodecahedron Fe / Zn-ZIF-8 Zn(CH3COO)2·2H2O (2.80 g, 12.72 mmol) and Fe(OH)(CH3COO)2 (0.20 g, 1.03 mmol) were weighed and dissolved in 50 mL of water. After sonicating to form a homogeneous solution, the solution was poured into a 50 mL aqueous solution containing 2-methylimidazole (11.20 g, 136 mmol) and hexadecyltrimethylammonium bromide (CTAB) (10.0 mg, 0.028 mmol). The solution was magnetically stirred at room temperature until it changed from clear to turbid. The stirring time was about 1 minute. After standing for 2 hours, Fe / Zn-ZIF-8 nanoparticles were obtained by centrifugation and washing with water 3 times.

[0036] (2) Preparation of layered Fe / Zn-ZIF-8@DAP array structure The 300 mg Fe / Zn-ZIF-8 nanoparticles obtained in step (1) were dispersed in 30 mL of water, and 15 mg of diammonium hydrogen phosphate (DAP) was added. After ultrasonication to form a stable mixed colloidal solution, the solution was rapidly frozen with liquid nitrogen and then transferred to a freeze dryer for freeze drying, finally obtaining the precursor of the layered Fe / Zn-ZIF-8@DAP array structure.

[0037] (3) Preparation of hollow carbon nanocage array catalysts supported on P-doped Fe single atoms The precursor obtained in step (2) was placed in a tube furnace, and nitrogen gas was introduced into the tube furnace at a flow rate of 100 mL / min. Under inert gas protection, the temperature was increased to 900 °C at a heating rate of 3 °C / min, and then calcined at a constant temperature for 2 hours. The temperature was then decreased at a cooling rate of 10 °C / min until it was cooled to room temperature, resulting in a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms. Its microstructure is as follows: Figure 1 As shown, the TEM image clearly reveals the morphology of the carbon nanocage layers, with each nanocage having 12 carbon pillars and accompanied by highly developed hollow features.

[0038] Figure 2 The image shows a HAADF-STEM image of the hollow carbon nanocage array catalyst loaded with P-doped Fe single atoms prepared in this embodiment. The bright spots in the image demonstrate the presence of Fe single atoms.

[0039] Figure 3 It is a thermogravimetric curve, from Figure 3 It is evident that DAP has poor thermal stability, exhibiting a rapid decomposition initiation at approximately 135 °C, while simultaneously generating ammonia gas at low temperatures (the principle is as follows). Figure 4(As shown). Without DAP doping, Fe / Zn-ZIF-8 exhibits strong thermal stability, retaining 99.52% of its initial mass after heating to 250 °C. However, with the addition of DAP, the net mass loss percentage of Fe / Zn-ZIF-8 increases significantly, indicating that DAP has a significant etching effect on the ZIF-8 crystal structure. A first thermal plateau occurs at approximately 250 °C, suggesting that etching primarily occurs at this stage.

[0040] like Figure 5 As shown in Figure a, the evolution of the carbon nanocage structure can be divided into two stages: anisotropic etching (stage 1) and stress-induced shrinkage-driven void formation (stage 2). The structural evolution process was monitored by temperature-dependent pyrolysis. Corresponding to Figure a, the initial morphology of the uncooked Fe / Zn-ZIF-8@DAP particles was a solid, truncated rhombic dodecahedron (e.g., ...). Figure 5 As shown in Figure b1, during pyrolysis at 400 °C, a small pore was observed in the central region of the {100} facet of the ZIF-8 nanopolyhedron, while the {110} facet remained intact (as shown in Figure b1). Figure 5 (As shown in Figure b2). Subsequently, at 500 °C, the hole further enlarged, indicating that the crystal structure was partially damaged (as shown in Figure b2). Figure 5 (As shown in Figure b3). When the temperature is further increased to 600 ℃, the crystal structure of ZIF-8 begins to collapse, the carbonization process begins, indicating that the chemical etching is complete and stage 2 begins (as shown in Figure b3). Figure 5 (As shown in Figure b4). In stage 2, as the temperature increases, the pores at the center of the {100} plane gradually expand, forming the final nanocage structure (as shown in Figure b4). Figure 5 (As shown in Figure b5). The intact {110} plane of ZIF-8 transforms into twelve highly rigid carbon pillars. These rigid pillars generate a specific outward contraction stress, and ZIF-8 decomposes from the center of the damaged {100} lattice towards the rigid pillars, ultimately forming an open nanocage array structure. This phenomenon can be attributed to the ammonia gas released at lower temperatures during DAP decomposition, which acts as an etchant for ZIF-8. This is because the Zn-2-MIM bond density in the {100} plane of ZIF-8 is higher than that in the {110} plane (as shown in Figure b5). Figure 5 (As shown in Figure c). The lone pair of electrons on the nitrogen atom in the ammonia molecule acts as a Lewis base, attacking the Zn-2-MIM bond (as shown in Figure c). Figure 6As shown in the figure, this leads to the breaking of coordination bonds and the structural collapse of the ZIF-8 crystal. Therefore, the denser Zn-2-MIM bonds in the {100} surface and along the […] <100> The higher etching rate observed in the direction compared to along <110> The etch rate observed in the direction was low. Notably, in addition to ammonia release, the decomposition of DAP also produces phosphorus-containing organic matter, which can enable the modulation of the single-atom coordination environment of the metal.

[0041] Figure 7 The N2 adsorption isotherms of the solid carbon nanoparticle array obtained in step (2) and the hollow carbon nanocage array catalyst obtained in step (3) in this embodiment are shown below. Figure 7 As can be seen, the hollow carbon nanocage array exhibits a distinct hysteresis loop type IV isotherm, indicating that it possesses a hierarchical porous structure, containing micropores, mesopores, and macropores simultaneously. The calculated specific surface area reaches as high as 1470 m². 2 / g, pore volume is 2.14 cm³ 3 / g. This unique hierarchical porous honeycomb carbon significantly improves macroscopic mass transfer efficiency, while the hollow interior further accelerates the mass transfer rate and improves the accessibility of active sites.

[0042] The hollow carbon nanocage array catalyst with P-doped Fe single atoms prepared in this embodiment was used for electrocatalytic ORR testing. The testing method is as follows: ORR performance was measured in a conventional three-electrode system on a CHI 760E electrochemical workstation. The electrolyte consisted of 0.1 mol / L potassium hydroxide. The reference electrode and counter electrode were Ag / AgCl (3 mol / L potassium chloride) electrode and carbon rod, respectively. A rotating disk electrode (RDE) with a diameter of 3 mm was used as the substrate for the working electrode. 8 μL of catalyst ink was dropped onto the working electrode and allowed to dry naturally, with a mass loading of 0.57 mg / cm³. 2 Before measurement, the electrolyte should be purged with oxygen for at least 30 minutes.

[0043] The results are as follows Figure 9 As shown in the LSV diagram, the hollow carbon nanocage array catalyst prepared in this embodiment has excellent electrocatalytic performance compared with the solid carbon nanoparticle array. Its ORR half-wave potential reaches 0.920 V, which is much higher than that of the solid carbon nanoparticle array (0.874 V). Weigh 3 mg of the solid carbon nanoparticle array obtained in step (2) and the hollow carbon nanocage array catalyst obtained in step (3), respectively, and mix them with 25 μL of 5% Nafion solution and 575 μL of ethanol. After ultrasonic homogenization, the mixture is uniformly drop-coated onto carbon paper to obtain an air cathode with a mass loading of 1 mg / cm³. 2A liquid ZABs battery was prepared using a Zn sheet as the anode and a 6 M KOH + 0.2 M zinc acetate aqueous solution as the electrolyte. The discharge and power density of the liquid ZABs battery were then tested using the following methods: Before performance testing, the electrolyte needs to be oxygenated for 30 minutes to ensure it is fully saturated with oxygen. Discharge polarization curves were recorded using a CHI 760E electrochemical workstation. Linear sweep voltammetry (LSV) was performed with a Zn electrode as the negative electrode and an air electrode as the positive electrode. The sweep voltage range was 0–1.5 V, and the sweep rate was 50 mV / s.

[0044] The results are as follows Figure 10 As shown, it exhibits a power density far superior to solid carbon nanoparticle arrays and commercial Pt / C cathodes.

[0045] PEMFCs were prepared using the solid carbon nanoparticle array obtained in step (2) and the hollow carbon nanocage array catalyst obtained in step (3), respectively. The discharge and power density of the PEMFCs were then tested. The methods are as follows: The electrochemical performance of PEMFCs was tested in an 850e fuel cell test system (manufactured by Scribner Associates Inc.). PEMFCs with an effective area of ​​4 square centimeters were prepared using a spray deposition method. The proton exchange membrane, Nafion 212 (purchased from Fuel Cell Stores), was treated with aqueous solutions containing 5% (w / w) H₂O₂ and 0.5 mol / L H₂SO₄ in an 80 °C water bath for 1 hour, followed by rinsing with deionized water after each treatment. A catalyst slurry was prepared by ultrasonically mixing 20% ​​Pt / C (20 mg), isopropanol (4 mL), deionized water (12 mL), and Nafion solution (100 mL), serving as the cathode material. The anode side used a commercial 60% Pt / C catalyst, with the Pt loading reaching 1.0 mg / cm² achieved through spray deposition. The gas diffusion layer (YLS-30T) was purchased from Fuel Cell Stores and was not further treated before use. During the test, the flow rates of hydrogen, oxygen, and air (21% oxygen and 79% nitrogen) were all 300 mL / min. The test conditions were controlled at 80 ℃ and 100% relative humidity, with no back pressure.

[0046] The results are as follows Figure 11 As shown, the discharge performance of the hollow carbon nanocage array catalyst is significantly better than that of the solid carbon nanoparticle array.

[0047] Example 2: A method for preparing a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms is as follows: (1) Preparation of cubic Fe / Zn-ZIF-8 (C-Fe / Zn-ZIF-8) Zn(CH3COO)2·2H2O (2.80 g, 12.72 mmol) and Fe(OH)(CH3COO)2 (0.20 g, 1.03 mmol) were weighed and dissolved in 50 mL of water. After sonicating to form a homogeneous solution, the solution was poured into a 50 mL aqueous solution containing 2-methylimidazole (6.48 g, 78 mmol) and hexadecyltrimethylammonium bromide (CTAB) (20.0 mg, 0.056 mmol). The solution was magnetically stirred at room temperature until it changed from clear to turbid, for about 1 minute. After standing for 2 hours, cubic C-Fe / Zn-ZIF-8 nanoparticles were obtained by centrifugation and washing with water three times.

[0048] (2) Preparation of layered C-Fe / Zn-ZIF-8@DAP array structure The 300 mg C-Fe / Zn-ZIF-8 nanoparticles obtained in step (1) were dispersed in 30 mL of water, 15 mg of DAP was added, and after sonication to form a stable mixed colloidal solution, the solution was rapidly frozen with liquid nitrogen and then transferred to a freeze dryer for freeze drying, finally obtaining the precursor of the layered C-Fe / Zn-ZIF-8@DAP array structure.

[0049] (3) Hollow carbon nanocage array catalyst supported on P-doped Fe single atoms The layered C-Fe / Zn-ZIF-8@DAP array precursor obtained in step (2) was placed in a tube furnace, and argon gas was introduced into the tube furnace at a flow rate of 30 mL / min. Under argon protection, the temperature was increased to 900 ℃ at a rate of 4 ℃ / min, and then calcined at a constant temperature for 3 hours. The temperature was then decreased at a rate of 10 ℃ / min until it was cooled to room temperature, resulting in a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms, the morphology of which is as follows. Figure 5 As shown in Figure d3, the SEM image clearly reveals the morphology of the carbon nanocage layers, accompanied by highly developed hollow features. Because the C-Fe / Zn-ZIF-8 crystal consists of six {100} facets, under pyrolysis conditions of 900 ℃, the Zn-2-MIM bonds in all {100} facets of C-Fe / Zn-ZIF-8 are strengthened by the etching action of NH3, and all {100} facets are completely hollowed out, forming cubic carbon nanocages (such as...). Figure 5 (As shown in Figures d1 and d2). However, unlike the highly rigid cylinders derived from ZIF-8, the twelve thin edges in the C-Fe / Zn-ZIF-8 derived carbon nanocages exhibit lower rigidity, leading to varying degrees of structural deformation.

[0050] Tests showed that the electrocatalytic performance and discharge performance of the catalyst final product obtained in this embodiment were similar to those of the catalyst obtained in Example 1.

[0051] Example 3: A method for preparing a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms is as follows: (1) Preparation of rhombic dodecahedral Fe / Zn-ZIF-8 (R-Fe / Zn-ZIF-8) Zn(CH3COO)2·2H2O (2.80 g, 12.72 mmol) and Fe(OH)(CH3COO)2 (0.20 g, 1.03 mmol) were weighed and dissolved in 50 mL of water. After sonicating to form a homogeneous solution, the solution was poured into 50 mL of aqueous solution containing 2-methylimidazole (11.20 g, 68 mmol). The solution was magnetically stirred at room temperature until it changed from clear to turbid, for about 1 minute. After standing for 2 hours, cubic R-Fe / Zn-ZIF-8 nanoparticles were obtained by centrifugation and washing with water three times.

[0052] (2) Preparation of layered R-Fe / Zn-ZIF-8@DAP array structure The 300 mg R-Fe / Zn-ZIF-8 nanoparticles obtained in step (1) were dispersed in 30 mL of water, 15 mg of DAP was added, and after sonication to form a stable mixed colloidal solution, the solution was rapidly frozen with liquid nitrogen and then transferred to a freeze dryer for freeze drying, finally obtaining the precursor of the layered R-Fe / Zn-ZIF-8@DAP array structure.

[0053] (3) Hollow carbon nanocage array catalyst supported on P-doped Fe single atoms The layered R-Fe / Zn-ZIF-8@DAP array precursor obtained in step (2) was placed in a tube furnace, and nitrogen gas was introduced into the tube furnace at a flow rate of 60 mL / min. Under nitrogen protection, the temperature was increased to 900 °C at a rate of 2 °C / min, and then calcined at a constant temperature for 2 hours. The temperature was then decreased at a rate of 5 °C / min until it was cooled to room temperature, resulting in a hollow carbon nanocage array catalyst supported on P-doped Fe single atoms. Its microstructure is as follows: Figure 5 As shown in Figure e3.

[0054] Figure 5 Figures e1 and e2 show that the R-Fe / Zn-ZIF-8 crystal contains {110} facets, {100} and {111} vertices, and {211} edges. The {100} vertex, with its denser Zn-2-MIM bonds, and its four adjacent {211} edges, exhibit a preferential etching tendency, which effectively proves the theory of NH3 selective etching.

[0055] Tests showed that the electrocatalytic performance and discharge performance of the catalyst final product obtained in this embodiment were similar to those of the catalyst obtained in Example 1.

[0056] Example 4: A method for preparing a hollow carbon nanocage array catalyst supported on P-doped Co single atoms is as follows: (1) Preparation of tangential rhombic dodecahedron Co / Zn-ZIF-8 Zn(CH3COO)2·2H2O (2.80 g, 12.72 mmol) and Co(CH3COO)2·4H2O (0.20 g, 0.92 mmol) were weighed and dissolved in 50 mL of water. After sonicating to form a homogeneous solution, the solution was poured into a 50 mL aqueous solution containing 2-methylimidazole (11.20 g, 68 mmol) and hexadecyltrimethylammonium bromide (CTAB) (10.0 mg, 0.028 mmol). The solution was magnetically stirred until it changed from clear to turbid, for about 1 minute. After standing for 2 hours, Co / Zn-ZIF-8 nanoparticles were obtained by centrifugation and washing with water three times.

[0057] (2) Fabrication of layered Co / Zn-ZIF-8@DAP array structure The 300 mg Co / Zn-ZIF-8 nanoparticles obtained in step (1) were dispersed in 30 mL of water, 15 mg of DAP was added, and after sonication to form a stable mixed colloidal solution, the solution was rapidly frozen with liquid nitrogen and then transferred to a freeze dryer for freeze drying, finally obtaining the precursor of the layered Co / Zn-ZIF-8@DAP array structure.

[0058] (3) Hollow carbon nanocage array catalyst supported on P-doped Co single atoms The layered Co / Zn-ZIF-8@DAP array precursor obtained in step (2) was placed in a tube furnace and heated to 900 ℃ at a rate of 5 ℃ / min under argon protection. It was then calcined at a constant temperature for 4 hours and cooled to room temperature at a rate of 10 ℃ / min to obtain a hollow carbon nanocage array catalyst loaded with P-doped Co single atoms.

[0059] Tests showed that the electrocatalytic performance and discharge performance of the catalyst final product obtained in this embodiment were similar to those of the catalyst obtained in Example 1.

[0060] Example 5: Everything else is the same as in Example 1, except that: In step (1), Fe(OH)(CH3COO)2 is replaced with copper acetate.

[0061] In step (2), DAP is replaced with ammonium dihydrogen phosphate, and the mass ratio of Cu / Zn-ZIF nanoparticles to ammonium dihydrogen phosphate is 300:10.

[0062] Example 6: Everything else is the same as in Example 1, except that: In step (2), the mass ratio of Fe / / Zn-ZIF to DAP is 300:20.

[0063] Comparative Example 1: Everything else is the same as in Example 1, except that: In step (2), 300 mg of Fe / Zn-ZIF nanoparticles were dispersed in 30 mL of water without the addition of DAP. After the Fe / Zn-ZIF solution was sonicated to form a stable mixed colloidal solution, it was rapidly frozen with liquid nitrogen and then transferred to a freeze dryer for freeze drying. Finally, a precursor with a layered Fe / Zn-ZIF-8 array structure was obtained. The precursor was then processed in step (3) to obtain the final product, the microstructure of which is as follows. Figure 8 As shown, without the addition of DAP, ZIF-8 cannot be etched. Therefore, the resulting carbon nanoparticle product retains the chamfered rhombic dodecahedral morphology of ZIF-8, forming a solid carbon nanoparticle array.

Claims

1. A catalyst for oxygen reduction reaction, characterized in that, This includes layered structures formed by arrays of hollow carbon nanocages loaded with P-doped metal single atoms.

2. The oxygen reduction reaction catalyst according to claim 1, characterized in that, The metal includes at least one of Co, Fe, and Cu, and the carbon nanocage is in the shape of a cube, a rhombic dodecahedron, or a chamfered rhombic dodecahedron. The hollow carbon nanocage contains micropores, mesopores, and macropores.

3. The method for preparing the oxygen reduction reaction catalyst according to claim 1, characterized in that, Includes the following steps: (1) Dissolve zinc salt and second metal salt in water to obtain metal source solution, and then react the metal source solution with 2-methylimidazole aqueous solution to obtain metal-organic framework nanoparticles; (2) Disperse metal-organic framework nanoparticles and phosphorus-containing etchant that can thermally decompose to generate ammonia in water to form a mixed colloidal solution, and freeze-dry the mixed colloidal solution to obtain a layered precursor; (3) The oxygen reduction reaction catalyst is obtained by calcining the layered precursor.

4. The method for preparing the oxygen reduction reaction catalyst according to claim 3, characterized in that, In step (1), the second metal salt is at least one of the acetates of Co, Fe, and Cu, the zinc salt is zinc acetate, and the 2-methylimidazole aqueous solution is an aqueous solution containing 2-methylimidazole and a quaternary ammonium salt surfactant.

5. The method for preparing the oxygen reduction reaction catalyst according to claim 4, characterized in that, The molar ratio of zinc, the second metal, and 2-methylimidazole in the metal source solution is 10-15. 0.5-1.5：50-150。 6. The method for preparing the oxygen reduction reaction catalyst according to claim 3, characterized in that, In step (2), the phosphorus-containing etchant includes at least one of diammonium hydrogen phosphate and diammonium dihydrogen phosphate.

7. The method for preparing the oxygen reduction reaction catalyst according to claim 3, characterized in that, In step (2), the mass ratio of metal-organic framework nanoparticles to phosphorus-containing etchant is 200-400:5-25; In step (3), the calcination conditions are: under an inert atmosphere, the temperature is raised to 500-1000 ℃, calcined at a constant temperature for at least 2 hours, and then cooled to room temperature.

8. The method for preparing the oxygen reduction reaction catalyst according to claim 7, characterized in that, The heating rate is 1-5 °C / min, and the cooling rate is 5-10 °C / min.

9. The application of the oxygen reduction reaction catalyst according to claim 1 or 2 in electrochemical oxygen reduction testing.

10. The application of the oxygen reduction reaction catalyst according to claim 1 or 2 in the preparation of battery cathode.

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